A diamond is a natural crystal - an allotrope of carbon. Its chemical composition consists of 100% carbon atoms, arranged in an isometric - hex-octahedral crystal lattice: Each atom is linked to four other carbon atoms by 4 valence electrons, creating the stable - hard cubic structure.
The fact that diamonds belong to the cubic crystal system, doesn't mean that the outward shape of a diamond will necessarily be a cube. In fact, diamonds can have many facets that belong to a cube, octahedron or dodecahedron.Although diamonds most often occur in their octahedron shape, cube shaped or dodecahedron shaped diamonds can also be found, although they are less common and very rare.The outward ultimate shape depends on various external circumstances, such as temperature and pressure.
Theories of Diamonds Formation Process:
The first diamonds were discovered at the bottom of old and existing river-beds, but, in the end of the 19th century, diamonds were also discovered in large stone pipes, which had extend downwards into the depth of the earth. The material of those pipes was called "Kimberlite" or"lamproite".
Kimberlite was first considered to be either hardened lava or the raw material of which extinct volcanoes were compounded. Therefore people assumed it was the material which created diamonds.
There are three main theories which relate to the formation process of a diamond:
One theory concludes that fluids enriched with water, carbon monoxide, nickel, iron and copper are all components that are essential to the diamond's formation. Some claim, making an analogy to the process of synthetic diamond's manufacture, that it is the transition of sulfur into a diamond. Other conjecture suggests that it is carbonic fluid or gases that are the diamond's origin.
A different theory concludes, that diamond "seeds" have been created in pools located in the crust of the earth and that these seeds have grown due to accumulation of increasing number of carbonic atoms to the "seed's" external shells. This external shell, according to this theory, is formed by carbon monoxide or by an oxidation of methane gas, which is believed to be the cause for elevating the kimberlite material close to the earth's surface.
Another theory suggests that the methane gas which is found in the magma is the basic material of the diamond (and not the carbon monoxide). This gas is the simplest form of carbonic material; it can be detached from the magma at high temperatures and pressures conditions. After the magma cools, the diamond's formation process would be possible (assuming the proper conditions which have been mentioned above occur).
1] The magma is a complex high-temperature fluid substance, which is actually a molten rock which is located beneath the surface of the Earth.